1. Field of the Invention
The present invention relates to a system for measuring a distance to an object to be measured and more particularly to an active distance measurement system favorably applied to various types of cameras.
2. Related Background Art
Such an active distance measurement system applied to cameras generally includes an infrared-emitting diode (IRED) for emitting an infrared beam toward an object to be measured, and a position sensitive detector (PSD) for receiving the object-reflected infrared beam. The signal output from the PSD is a signal responsive to a position where the object-reflected infrared beam is received. A signal processing and arithmetic unit determines a distance to the object to be measured from this signal. Because a large error may occur at once measurement, averaging of multiple pieces of distance information is generally performed to obtain more accurate distance information.
FIG. 4 shows a circuit diagram illustrating a configuration of an integrating unit used for obtaining the average of the distance information in the distance measurement system. This integrating unit 16 comprises a switch 1, an integrating capacitor 2, a switch 3, a constant current source 4, an operational amplifier 5, a switch 6, a reference power source 7, and a comparator 8. The negative input terminal of the operational amplifier 5 is connected through the switch 1 to the output terminal of an arithmetic unit 15 and grounded through the integrating capacitor 2. Furthermore the negative input terminal of the operational amplifier 5 is connected through the switch 3 to the constant current source 4, and connected through the switch 6 to the output terminal of the operational amplifier 5. Also, the positive input terminal of the operational amplifier 5 is connected to the reference power source 7, which provides a reference voltage V.sub.REF. The comparator 8 is connected to the junction between the negative terminal of the operational amplifier 5 and the integrating capacitor 2 and compares the potential of the junction and the reference voltage V.sub.REF to find out which is higher. The comparator 8 outputs a signal corresponding to the comparison results. A central processing unit (CPU) 19 receives the signal output from the comparator 8 and controls the on-off operation of the switches 1, 3 and 6.
As an example of the distance measurement system using such an integrating unit 16 is a distance measurement system mounted in a camera. When a shutter release button is half- or partially-depressed after powering on the camera, the CPU 19 turns on the switch 6 to charge the integrating capacitor 2. As the result, the integrating capacitor 2 is charged, as generally shown in FIG. 5, to the reference voltage V.sub.REF provided by the reference power source 7. After the charging up, the switch 6 is turned off and retained in such a state.
Then, the IRED emits infrared pulses and the switch 1 is turned on. As a result, output signals (distance information) from the arithmetic unit 15 are input into the integrating capacitor 2 asnegative voltages. AS shown in FIG. 5, the voltage across the integrating capacitor 2 decrementally changes step by step in value corresponding to each distance measurement information. This is called a "first integrating".
After the predetermined number (e.g., 256) of negative voltage inputs (discharges) into the integrating capacitor 2 are completed, the switch 1 is turned off and the switch 3 is turned on in response to control signals from the CPU, whereby the integrating capacitor 2 is charged at a fixed speed defined by the power rating of the constant current source 4. This is called a "second integrating".
All the while of the second integrating, the comparator 8 compares the voltage level of the integrating capacitor 2 and the reference voltage V.sub.REF. If the comparator 8 estimates that they are coincident with each other then the comparator 8 turns the switch 3 off to stop charging the integrating capacitor 2,i.e. finish the second integrating. The CPU 19 counts a charging time of capacitor 2 (length of time spent in the second integrating). As the charging speed by the constant current source 4 is uniform, the sum of the signal voltages input into the integrating capacitor 2 during the first integrating can be determined from the aforementioned charging time of capacitor 2. The distance to the object can be determined based on the resultant sum. On the basis of the obtained distance to the object, the CPU 19 controls a driving of lens to focus. In the subsequent distance measurement, as the required charging of the integrating capacitor 2 has been realized by the constant current source 4, the switch 3 may be retained open, unless the constant current source 4 is provided in use for a long time.